In the spawnFallingBlock method of CraftWorld the location that is accepted for creating the FallingSand entity is reset to the middle of the current block.

This does not allow developers to spawn falling sand which does not match the grid.

If for whatever reason you would like to maintain that it centres the blocks to the grid, there is also another issue with this as in at least newer minecraft versions is that the blocks actually spawn half a block above the block they were intended to be spawned in. This is due to falling sand being centred at the centre of the bottom face of the block, not the absolute centre of the block, which is something which may have been changed in newer versions of minecraft. To fix this you would just need to not add 0.5 to the y of the falling sand.